1. Field of the Invention
This invention relates to a magneto-optical recording medium wherein information bits (magnetic domains) are read out by the magneto-optical effect and more particularly, to a magneto-optical recording medium wherein information bits are partially distinguished or expanded depending on the temperature distribution in a reproduction beam spot at the time of reproduction. The invention also relates to reproduction methods using the medium mentioned above.
2. Description of the Prior Art
The magneto-optical recording system makes use of the fundamental principle that a magnetic thin film is partially heated to a temperature which exceeds the Curie point or compensation temperature so that the coercive force of the heated portion is lost and the direction of magnetization is inverted to conform to the direction of a recording magnetic field which is applied from outside. A known magneto-optical recording medium has such an arrangement which includes a transparent substrate such as of, for example, polycarbonate and a recording portion formed on one side of the substrate. The recording portion includes a built-up layer structure which has a magnetic recording layer, which has an axis of easy magnetization in a direction vertical to the film plane and good magneto-optical characteristics and is made, for example of a rare earth element-transition metal amorphous alloy, a reflection layer, and a dielectric film. Signals are read out by irradiation of a laser beam from the side of the transparent substrate.
Not only in the magneto-optical recording medium, but also in digital audio disks (so-called compact disks) or optical disks such as video disks, the linear recording density is determined by the S/N value at the time of reproduction. The quantity of reproduction signals depends greatly on the pitch or frequency of recorded signal bit rows and the laser beam wavelength and the numerical aperture of an objective lens of a reproducing optical system.
At present, when the laser beam wavelength, .lambda., and the numerical aperture, N.A. are determined, the bit frequency, f, which becomes a detection limit is determined as follows: f=.lambda./2 N.A.
Accordingly, in order to make high density recording of optical disks, it is fundamentally essential to shorten the laser beam wavelength .lambda., and increase the numerical aperture, N.A. of the objective lens.
However, existing techniques place limitations on the improvements in the laser beam wavelength .lambda., and the numerical aperture N.A. of the objective lens. Another approach to improve the recording density has been made wherein efforts have been made in the construction of the magneto-optical recording medium and the manner of reading out from the medium.
For instance, the present applicant proposed in Japanese Laid-open Patent Application Nos. 1-143041 and 1-143042, incorporated herein, systems wherein an information bit (magnetic domain) is partly expanded, shrunk or extinguished, thereby improving reproducing resolution. In the system, the magnetic recording layer is made of an exchange-interacted multi-layered film which includes a reproduction layer, an intermediate layer and a record-holding layer. At the time of reproduction, a magnetic domain of the reproduction layer which has been heated by means of a reproducing light beam is expanded or shrunk or extinguished at the high temperature portion, making it possible to reproduce a signal with a pitch which exceeds a diffraction limit.
In the procedure wherein information bits are partially expanded, shrunk or extinguished at the time of reproduction, record information with a density higher than a record density which is determined depending on the laser beam wavelength .lambda., and the numerical aperture N.A., can be reproduced. We made further investigations and, as a result, found that mere selection of the Curie point and the coercive force does not always lead to reproduction with a high C/N (Carrier-to-Noise) value.